The best answer is obviously the hearts in legend of zelda games. Because only eating the hearts of your enemies<fresh, or preserved and stored in jars and tufts of grass> should restore your health!
I don't like most uses for regenerating health, health regen in combat is stupid and no matter how much the gow people want to defend it, all it does is turn cover based games into whack a mole. There is no skill in ducking behind the wall to wait for full health if you did something stupid because all it allows you to do is do something stupid over and over again till you whittle down the enemy.
After combat, sure, it works great in games like GW2 because it allows the battles to be more close to the wire affairs since you can easily heal up between fights and it streamlines the typical MMO format of fighting and then sitting on the ground to heal....
After battle regen works in fps too because it allows the devs to make levels that don't scale down farther in to make up for potential damage you took earlier in the level.